(1) Field of the Invention
The present invention relates to suspension insulators, more particularly, to suspension insulators suitable to be used in direct current transmission lines.
(2) Background of the Invention
There are used only about 20 direct current transmission lines at present all over the world. However, the direct current transmission line often causes troubles in the flashover of insulator due to contamination thereof. The reason is as follows: The direct current transmission line has a low internal abnormal voltage, and therefore the external insulation of the line is mainly designed based on the withstand voltage of insulators against contamination. However, the direct current withstand voltage of contaminated insulators has not yet hitherto been fully investigated.
That is, in the design of conventional external insulation for direct current transmission line, the following methods have been used, that is, a method, wherein ordinary insulators are merely used, thereby the same surface leakage distance as that in the alternative current transmission is secured in the direct current transmission; a method, wherein the kind and number of insulators to be used are determined based on the assumption that the direct current withstand voltage of a contaminated insulator is the same as the alternative current withstand voltage thereof; and a method, wherein the direct current withstand voltage of a contaminated insulator is measured by the use of a weak test direct current supply source. However, a very important fact has recently been found out from the research for the troubles in the past and from the investigation by the use of a powerful test direct current supply source for a new important direct current transmission line. That is, when an insulator is contaminated in the same degree, the direct current withstand voltage of the contaminated insulator is lower than the alternative current withstand voltage thereof, and the difference between the voltages is larger as the insulator is contaminated more heavily. This is based on the reason that there is no alternation of voltage with respect to time in the direct current contrary to the alternative current, and when a local arc is once formed due to contamination, the arc is apt to grow. Under these circumstances, it has been eagerly demanded to develop an insulator having excellent dimension and size suitable for obtaining a high direct current withstand voltage against contamination.